Pine martens

If there was ever an animal that showed the changes that cascade through the web of ecological relationships when you add or remove one of its members, it is the pine marten (Martes martes). This native woodlander was once removed from most of the country by the usual suspects – persecution and habitat loss – but is now making a dramatic comeback, reintroducing itself to natural communities that it has been absent from for over a hundred years. The direct and knock-on effects of this return from internal exile are positive for some other species, negative for others, but overall the web of native life seems to be strengthened by the restoration of a missing thread.

The pine marten is a mustelid, a member of the family that includes weasels, stoats, badgers and otters, and it has the long, lithe shape of all the smaller members of that group. It is the only native member of the family that is suited to life in the trees – indeed no other Scottish predator is quite so adapted to tree climbing. Its adaptations include powerful forelimbs, a long, bushy tail that helps with balancing, and strong, semi-retractable claws. Pine martens are about the size of a domestic cat, with cappucino colouring: richly dark brown fur with a creamy white throat patch.

Six thousand years ago, when Britain was still largely forested, pine martens were one of the most common predators around, but human influence has been disastrous for them. The long, slow destruction of the forest gradually removed the pine marten’s habitat. Although they are capable hunters on the ground, martens rely on trees for escape from their own predators and will not live in deforested areas. Trapping for their fur, which is considered to be of high quality, also contributed to their decline.

Then, during the nineteenth century, pine martens were persecuted by game estates, determined to eradicate all non-paying predators. Martens are particularly vulnerable to persecution as they are slow breeders, with females not usually breeding until their third year. Litters are small, typically just one to three kittens. By the end of the nineteenth century, healthy pine marten populations in mainland Britain were confined to the north west highlands, where it is estimated that around 1,500 animals remained.

It is probably no coincidence that the low point for marten populations was just before the First World War, after which the game estates declined, at least in their ability to wage all-out war on ‘vermin’, and the Forestry Commission was set up to create a strategic reserve of timber – Britain having come close to losing the war for the lack of it.

Since then, things have been looking up for the pine marten, and in 1988 they were given full legal protection under the Wildlife and Countryside Act. In 2012, a new study1 investigated the extent of the marten’s recovery. It isn’t easy to take a census of such rare, secretive creatures, so the researchers collected scats (droppings) which were DNA-tested to distinguish pine marten from fox and other scats. Some of the surveyors were four-legged themselves: two specially trained dogs helped to sniff out the scats. Survey methods are improving all the time: DNA analysis can now distinguish between individual animals, giving a direct way of counting the number of martens in an area rather than having to try to infer it from scat numbers and territory sizes. The DNA is collected both from scats and from baited pipe traps which harvest a few hairs from the martens as they run through them.

The 2012 study found signs that pine martens have expanded right across the north-east of Scotland and down into Perthshire, Stirlingshire, Fife, Argyll and the northern Central Belt. So far, their border seems to be The Forth-Clyde corridor, but when I spoke to Rob Raynor, SNH’s mammals specialist, a sighting had just come in from just south of the Erskine Bridge, so pine martens in Paisley could be the next step. There is also a population spreading up from Dumfries and Galloway, dating from a reintroduction in the 1980s.

One man who has seen pine martens spread through his own territory is Dave Anderson, a conservation manager with the Forestry Commission. Dave works in the Cowall peninsula in Argyll and began to see pine martens from the year 2000. In particular, they started turning up in the nest boxes that he puts up to study tawny owls. For a pine marten, a bird box constitutes both bed and breakfast: they will predate the eggs and take over the boxes as a lie-up and breeding den. At the worst, thirty to forty percent of the boxes were being looted by pine martens, so Dave now protects them with pond liner stapled around the trunk of the tree to create a smooth, unclimbable surface.

One big effect that Dave has noticed has been on the squirrel population. Pine martens will eat grey squirrels but find it hard to catch the smaller red squirrel, which can escape out onto branches that are too thin to support a marten. This will have direct effects on the grey squirrel population, but animals at risk of predation are also at a disadvantage as they have to be much more cautious, leading to less success in feeding and breeding2. Since the introduced greys displace the native reds through direct competition and by carrying a virus that kills the reds within days, this bad news for the grey squirrel is good news for the red. Dave has seen grey squirrel numbers plummet and reds begin to return in the areas where pine martens have been turning up. His observations are supported by research from Ireland where it was noticed that red squirrels had extended their range in areas where pine martens were resurgent3.

This is an example of the pine marten having a positive impact on another native species, but there are also negative impacts. Pine martens will raid birds’ nests and have been known to take capercaillie eggs. As Rob Raynor was keen to point out, this does not necessarily translate into an effect at a population level and certainly isn’t a reason to resume pine marten persecution. And while a pine marten likes an egg if it can get it, they are not a major component of the pine marten diet, which in one study4 was found to consist almost entirely of voles (39%), berries (30%), and small birds (24%). Interestingly, as autumn came, the pine martens switched the bulk of their diet to fruit, even though vole populations also peak at this time. They are adaptable, and other studies have found deer carrion making up almost a third of their diet; they are also famous for their fondness for peanut butter and jam, which have been used to lure them to feeding stations in the Highlands.

Their liking for voles means that mature plantation forestry is surprisingly good for pine martens, offering a mix of tall trees for safety and vole-filled areas of clear-fell in which to feed. Younger plantations are not much use to pine martens as they lack den sites. In general, pine martens in Scotland use a greater diversity of den sites, including houses, nest boxes, burrows and rocky crevices, than in continental Europe, where most dens are in trees5. This is thought to be due to our general lack of large woodland trees but may also be because we have no black woodpeckers to make the large cavities that pine martens prefer.

Good den sites are important to pine martens not only because their long, thin shape is poor at keeping in heat, but also to keep them safe from their enemies, for pine martens are prey as well as predator. Foxes are their main predator at present, but Dave Anderson points out that originally they would have had to look out for lynx, eagle owls, golden eagles and goshawks too, all of which would impair the pine marten’s ability to hunt voles in open ground if they returned to their native ranges. For now though, the future of the pine marten looks bright and there are no obvious barriers to its expansion across the rest of the country.

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